Radio direction finder having sense antenna



Oct. 23, 1951 J. G. SPEER 2,572,211

RADIo DIRECTION FINDER HAVING sENsE ANTENNA Filed Jan. 12, 1948 Jose/obG. Sloeer ATTORNEYS.

Patented Oct. 23, 1951 RADIO DIRECTION FINDER HAVING SENSE ANTENNAJoseph G. Speer, Merriam, Kans., assigner of onehalf to H. G. Koenig,Merriam, Kans.

Application January 12, 1948, Serial No. 1,775

12 Claims.

This invention relates to the field of radio and particularly todirection finders, the primary object being to provide a device of suchcharacter as forms the 'subject matter of my co-pending application,Serial No. 707,696, led November 4, 1946, now Patent No. 2,490,660,granted December 6, 1949, this being a continuation in part that Visinherent in the device forming the subject matter of said earlier ledapplication.

Another important object of this invention is the provision of a radiodirection finder having a sense antenna wherein means is provided toimprove operating accuracy, making possible more correct bearings on themaximum, thereby permitting continuous aural monitoring and minimizinginterference from static and undesired radio signals.

Another important object of this invention is the provision of a radiodirection finder including amplifier stages, having new linearampliflcation characteristics, in both the laudio and rate of changechannels, thereby maintaining the audio output and the triggeringimpulses approximately uniform regardless of the amplitude of thereceived radio signal.

A further object of this invention is to provide a radio directionfinder having a voltage limiter in the rate of change channel thatlimits the triggering impulses to just above the value required foroperation, which together with pulse time delay, renders the directionfinder practically immune to bursts of static.

A still further object of this invention is the provision of a radiodirection finder having a limit type of automatic volume control that isineffective, throughout the change in signal level required for properfunctioning of the direction nder, but is effective at signal levelchanges in excess of these requirements.

Additional features and objects will be set forth and made clear duringthe course or" the following specification, reference being had to theaccompanying drawing showing a diagrammatical view of an electriccircuit forming a part 2 of a radio direction iinderhaving a senseantenna made in accordance with my present invention, and showingschematically the loop antenna, sense antenna, motor, motor shafts,gears, radio receiver and associated parts thereof.

In the drawing a directional antenna, constituting normally anelectrostatically shielded loop, is broadly designated by the numeral I0and the numeral I2 designates a non-directional or sense antenna.

Thewindings of the loop I0 are connected to a pair of collector rings I4and I6 and from the rings by means of contact brushes (not shown) to apair of wires i8 and 20. 'I'he received signal is conducted over thesewires to a radio receiver tuner amplifier 22 where it is amplified tothe level required for detection. No details of these receiver elementsare shown as they may be quite conventional.

The amplified radio signal is conducted through a transformer 24, whichis tuned by a pair of condensers 26 and 28, to a diode 32 of a vacuumtube 30. The radio frequency signal is rectified by the diode 32 and aresultant direct current voltage appears across a resistor 34, togetherwith a superimposed audio frequency signal if the radio signal wasmodulated. The radio frequency component is greatly attenuated by acondenser 36 whose value is so chosen as to cause inappreciableattenuation of the audio frequency component. These voltages areconducted through a resistor 38 to the grid 39 of vacuum tube 30. Acondenser 40 contributes almost complete radio frequency attenuation andlarge audio frequency attenuation.

'Ihe negative direct current voltage applied to the grid 39 of thevacuum tube 30 increases its grid bias and hence reduces its platecurrent. The positive cathode potential as produced by the plate currentflowing through a pair of resistors 52 and 54 consequently decreases asthe incoming signal increases.

The audio frequency voltages appearing across the resistor 34 areconducted through a resistor 42 and a condenser 48 to the grid 49 of avacuum tube 56. The condenser 48 blocks the direct current voltage fromthe grid 49. The resistor 42, a resistor 44 and a condenser 45 provideadditional, almost complete attenuation of the radio frequency componentand some uniform audio frequency attenuation.

The grid 49 of vacuum tube 56 is positively biased by reason of thevoltage drop across the resistor 54. Therefore the cathode of tube 56should be also positive and this is accomplished by means of a resistor58.

The voltage drop across the resistor 54, and hence the Agrid bias of thevacuum tube 56, changes with signal level. If the values of theresistors 54 and 58 are suitably chosen, the change in amplication ofthe vacuum tube 56 will vary approximately inversely as the signallevel. By this means the audible output may be held approximatelyconstant.

The audio signal as amplified by the vacuum tube 56 is conducted fromits plate over a wire 60 and through suitable coupling means (not hereshown) to subsequent conventional audio amplication as required foraural indications.

The voltage at the cathode of the vacuum tube 30, which rises and fallsas the signal decreases and increases respectively, is impressed uponthe grid 61 of a vacuum tube 68 over a wire 66.

The cathode vof the vacuum tube 68 must be more positive than the grid61 throughout the operating range of signal levels. This positive biasmay be conveniently secured from a usual storage battery 64 which inmost aircraft is 12 volts or more. Noise and ripple may be removed by achoke 72 and a condenser 14.

For the vacuum tube 6B, one of suitable characteristics should bechosen, and suitably loaded by a resistor 'l0 of proper value. This maybe accomplished so that, to a fair approximation, the absolute voltagechange at the plate of tube 68 is proportional to the percentage voltagechange at the gridY |51.l Thus the output voltage change is independentof signal level.

Now as the signal rises and falls the voltage at the plate of the vacuumtube 68 rises and falls. This rise or fall of plate voltage causes acharging current to flow through a condenser 16 and a resistor 'I8 toground. A voltage is therefore, produced across the resistor 78 that isproportional to the rate of rise or fall.

If the signal and Vplate voltage of the vacuum tube 68 are rising, thevoltage across the resistor 18 is positive. Thisv causesno significantaction. The loop antenna I is presumably rotating towards a position ofmaximum response.

After the loop |0 rotates beyond the point of maximum response, thesignal begins to fall. The percentage rate of fall is proportional tothe product of the rotative speed of loop antenna |0 and the tangent ofits angle of departure from the point of maximum response.

Ihis produces a negative voltage across the resistor 18. Assuming aconstant loop rotative speed, the magnitude of this voltage isYproportional to the tangent of the angular departure of loop |0 frommaximum and, as we have seen, is nearly independent of the strength ofthe received signal.

This negative voltage is conducted through a resistor 90 to the grid 9|of a vacuum tube 96 causing its plate current to decrease, and its platevoltage, because of the decreased drop in a resistor 98 to increase.

After these voltages attain a certain value, subject to a slight delayin time because of a pair of condensers 92 and |00, the first triggertubes consisting of a pair of vacuum tubes |02 and |04 are tripped inthe following manner:

The vacuum tube |04 is normally operated with its platecurrent'saturated. Its grid is at cathode potential and is onlyprevented from going substantially positive, by virtue of the limitingaction of a, pair of resistors ||2 and I4. This current flowing througha resistor |06 biases the cathode of the vacuum tube |02 to a pointconsiderably more positive than its grid, thus holding its plate currentextinguished.

The plate of the vacuum tube 96 is coupled through a resistor to thegrid of the vacuum tube |02. Therefore, the rising potential at theplate of the vacuum tube 96 is carried to the grid of the vacuum tube|02 so that it soon enters the conducting region at which point theplate current ilowing through a resistor |08 causes the plate potentialof the vacuum tube |02 to fall. As this action continues, this platevoltage soon falls to the point where the potential distribution acrossthe resistors ||2 and ||4 is such as to bring the grid potential of thevacuum tube |04 below its cathode potential. This causes a decrease inits plate current and hence a decrease in drop across the resistor |06.

A resistor 0 is made very much smaller than the resistor |08. Alsobecause of the amplification of the vacuum tube |02 its grid changespotential less than that of the vacuum tube |04. Both of these effectscause the decreasingl current of the vacuum tube |04 to over-ride theincreasing current of the vacuum tube decreases and the cathode of theVacuum tube |02 becomes less positive. But the grid 'potential has beenmade more positive by the tripping signal so it is evident that the'eiects just described effectively increase the exciting voltage once thetripping potentialhas been reached.

In fact, the currents promptly surge so that the plate current of thevacuum tubes |02 and |04 reach saturation and extinction respectively.This surge is accentuated by the charging current through the condenser|00 so that the action is practically vinstantaneous regardless of therate at which the tripping voltagev rises. This new condition ofequilibrium continues as long as the tripping voltage is maintained. Ifthe rate of change of the signal should decrease or cease this trippingvoltage will fall and at a definite release value surges will take placein a reversed manner and the circuit will restore to normal. Y

This releasing voltage is somewhat lower in magnitude than the trippingvoltage. This 'is an advantage as it prevents' momentary surges fromcausing multiple trips.

The instant the vacuum tubes |02 and |04 are tripped and prior to theirrelease, a second trigger circuit is actuated which controls thedirection of rotation of the loop antenna |0.

This trigger circuit consists of a pair of Vacuum tubes ||6 and H3 inthe respective plate circuits of which are a pair of fields |20 and |22for an actuating motor |24 for antenna |01 The armature of the motor |24is continuously supplied with power so that the motor |24 runscontinuously in one direction or the other dependingV upon which of thetwo field windings |20 or |22 are receiving current.

This second trigger circuit, like the first is unstable, unless theplate current of one of the two tubes IIS or ||8 is extinguished. Inthis condition the circuit is stable until, being tripped by the rsttrigger circuit, the current in the conducting'vacuum tube ||6 or ||8 isextinguished and the current rises to normal in the vacuum tube ||E or||8 that was extinguished. This switches the elds |20 and |22 of therotating motor |24 causing it and the loop I0 to reverse direction ofrotation.

|02.V Therefore, the net drop across the resistor |06 The grid of vacuumtube [I6 is held at a denite potential by the voltage divider consistingof a pair of' resistors |3 and |132. An identical voltage dividerconsisting of a pair ofv resistors Iil and ISl determines the gridpotential of the vacuum tube II8. Note that in each case the voltagedividers extend from a ground to the plate of the opposite vacuum tube II5 or I I8,

The plate voltage of the extinguished vacuum tube I|6 or IIB is muchhigher than that ofthe one carrying normal plate current. This isbecause of the absence of voltage drop across the motor eld or |22through which plate currents are drawn. Therefore, each tube IIG and IIBmaintains a suitable grid potential for the opposite tube. Each gridpotential is positive but that of the extinguished tube is substantiallyless so. The cat-bodes are, therefore, positively biased by a commoncathode. resistor |38 so that the relative grid potential of theextinguished vacuum tube is substantially negative and well below theextinction point. The relative grid bias of the conducting vacuum tubeis also negative but only by a moderate amount so that its plate currentis responsive to any change introduced at the grid.

The grids of the vacuum tubes I I8 and H8' are connected through a pairof condensers 26 and |28 to the plate of the Vacuum tube l. This platevoltage drops suddenly when the first trigger circuit is tripped. Thisnegative pulse is impressed on the grids of the vacuum tubes H and IIB.This, directly,L causes no signicant change in the plate current of theextinguished vacuum tube. The plate current of the other vacuum tube ishowever, suddenly dropped towards extinction. This causes a reduction ofcurrent through the resistor |38 and hence a reduction incathodepotential. Likewise, the fall in plate current of the con ductingvacuum tube causes a rise in its plate voltff r age. This rise, reducedin magnitude, is transmitted to the grid of extinguished vacuum tubewhere it overcomes the negative pulse potential as received from the rsttrigger circuit. Plate current now begins to flow producing a platevoltage change that transmitted to the other grid. accentuates thechange.

The conducting condition of the two vacuum tubes I I6 and I I8 is nowreversed. Likewise there has been an interchange of current between themotor fields |253 and |22. This causes the motor |24 to stop andreverse. This motor 24 causes the loop It to rotate through suitablereduction gears such as |40 and |42. Consequently the rotation of loopID also reverses` direction and now moves towards the point of maximumresponse with a resulting rise in signal strength,

This rise acting through the rate of change circuits causes the firsttrigger circuit to release, if it has not already done so. This producesa positive pulse through the condensers I 25 and |28 to the secondtrigger circuit. This produces no significant action and the pulsepromptly decays.

The loop antenna I Il continues to rotate, crosses the maximum to theregion of falling response. When it reaches the point where the rate offall is of the designed magnitude, the trip circuits again function andagain reverse the direction of rotation of loop I.

Thus it is evident that the loop will continue to oscillate to equalangular displacements each side of the point of maximum response. Theactual. bearing is of course its average position.

It is desirable that the relatively large loop oscillations be reducedin magnitude as indicated by an azimuthv indicator F46. To accomplishthisy a hunting integrator may be interposed in shafting |41 between theloop II] and the azimuth indicator |46..

Such an integrator forms the subject matter of a. pending applicationfor U. S. Letters Patent, Serial No. 707.730 led November 4, 1946,k nowPatent No. 2,447,171. This is symbolically shown as. indicated by thenumeral" |44.

In the above description several important features have been omitted asnot being absolutely essential to the operation of the direction finder.These features do contribute materially to its operation in a reallypractical and satisfactory manner.

Such are. a pair of rectiers, indicated by the numerals 8l). and 82. Theconducting direction is in each case as indicated by the arrow. These.could be. thermionic. diodes but are more. oon;- veniently of thegermanium crystalA type.

By means of a voltage divider consisting of resistors d4', 86 and 38,the rectier 82 is biased negativelyI and the rectifier 8E! is biasedpositively with respect to the resistor '58,. This places a positivebias on the grid of they vacuum tubey 96 so. a resistor 94 isincorporated producing a positive cathode potential slightly greaterthan that o the grid.

As previously explained a definite negative volt.- age is requiredacross the resistor 'I8 to trip the circuit. The bleeder current and theresistor 8B should be so proportioned that the drop across the resistorBIS is slightly inexcess of this voltage value.

The. rectifier 82 has little effect on voltages up to the trip value asthe back resistance. is very high. voltages appreciably above the tripvalue however, cause conduction in the rectier 82 S0. that these excessvoltages are substantially absorbed or clipped.

this marmerV a| noise or static surge is limited and of' short durationwill not trip the circuit due to the delay imposed by charging thecondenser 92 through the resistor 90 and the con denser Ittth-rough itsassociated resistors 98 andv III! together with plate resi-stance oftubes 96 and IM., Suitable time constants for these circuits areapproximately .0.5 second. The rectifier dii is positivelyy biased in asimilar manner and serves to clip positive voltage surges in the sameway'. The resisto-r 8.6 isv of the same magnitude as or' slightly lessthan the resistor` 88..

It is desirable that all audio modulation and other fluctuations. beremoved from the rectified carrier before transmittal. to the rate ofchange circuits. Now static noise bursts. may cause a rapid change incarrier strength. Such changes should be accommodated with little delaysothat they may be effectively clipped and delayed as previouslyexplained.

If long time constant. filters are usedv the eilectiveness of theclipping is. greatly reduced. For these reasons the resistor 38 and thecondenser 40 should have a time constant of about .01 second. A resistor|50 and a condenser |48 should have a time constant aboutv the same.

A resistor |54 and a condenser |52 which govern the screen voltage ofthe pentode 30, should have a time constant of about .02 second. Now thescreen contributes only about 20% of the current so the additional delaywill not cause any great direct change in the current rise or fall. Butthe plate and screen currents are but little affected by the platevoltage and greatly aieoted by the screen voltage. The effect is such asto resist change in current.

Therefore the effect of the relatively slow time constant lter on thescreen is to provide a delay in the restraint imposed by its voltagechange. This results in a somewhat higher initial change in totalcurrent which reaches its ultimate value without the excessive taperingoff that is usually characteristic of resistance capacity filters.

It was previously explained how the vacuum tubes 30, 56 and 68functioned so as to be independent of signal level variations. Now thisis true only throughout the limited operational range of the tubes inquestion and would by no means accommodate the signal strength ratioencountered in practice, which may be many thousandfold.

The need for some sort of volume control is evident. A simple manualvolume control would serve but its inconvenience would indicate thedesirability of an automatic volume control.

Now the conventional volume control tends to prevent change in therectified carrier, which is precisely the effect required in thisdirection finder. Such a volume control would require excessive timedelay in the automatic volume control circuits and excessive tripcircuit sensitivity.,V This is undesirable for several obvious reasons.

The automatic volume control employed, operates to reduce or increasethe amplification to any desired degree or at any desired speed.` Over-a limited range it is however, almost completely ineffective. It iswithin this range that all of the action previously explained takesplace.

Two rectiers are employed acting between an automatic volume controlbuss |66 and the cathode of vacuum tube 30. A rst rectifier |56 isconductive towards, and a second rectifier |58 is conductive away, fromthe cathode of vacuum tube 30.

At the initial no signal condition the cathode of vacuum tube 30 reachesa positive potential somewhat greater than that of the cathodes of thevacuum tubes being controlled. A bleeder current may be drawn from theplate supply source. to the cathode bias point through a pair ofresistors 50 and 5|. These resistors should be so proportioned that, attheir junction, the positive potential will equal the positive potentialof the cathode of vacuum tube 30 at the no signal condition.

` A resistor 6| should have about four times the resistance of aresistor |60. Now, as the signal increases, the cathode potential ofvacuum tube 30 decreases in positive potential. The positive cathodepotential of the rectier |56 likewise decreases but the decrease will beone-fifth smaller in magnitude.

As the incoming signal continues to increase. the cathode potential-ofthe rectifier |56 will eventually Vdrop below that of the automaticvolume control bus |66. When this takes place, the conductivity of therectifier |56 is such, that the AVC bus |66 will then have its positivepotential reduced. This rapidly d-ecreases the amplification of thereceiver until a balance is reached.

If the signal subsequently decreases, the cathode potentials of vacuumtube 30 and the rectier |56 rise. No immediate change occurs inpotential of the AVC bus |66, as the rectifier |56 is nownon-conductive. If the change continues the cathode potential of thevacuum tube Y 30 soon exceeds the potential of the AVC bus |66. At thispoint the rectifier |58 becomes conductive and the potential of Ythe AVCbus |66 rises with the potential of the cathode of the vacuum tube 30.This rapidly increases the amplification of the vacuum tube receiveruntil a balance is reached. i

This will permit a change in signal strength of any desired magnitudewithout automatic volume control action. If the resistors |60 and 6|lare proportioned in the ratio o f 1 to 4 a 20% drop in signal strengthis required to start AVC action. This should be adeguate to accommodatethe direction finding functions.

Amplification changes may be made at any desired speed by controllingthe charging rate of a condenser |64. If desired these speeds may differfor increases and decreases in amplification.

The condenser |64 is charged through a resistor |62 for increases inamplification and discharged through the parallel combination of theresistors |60 and |6| for decreases in amplif-lcation.

It is evident that, throughout all of the volume control action justdescribed, and for all operating conditions, the grids of all vacuumtubes beings controlled are at positive potential from ground.

It is necessary, however, that the receiver tubes being controlledoperate at all times with their grids negative with respect to theircathodes. It is, therefore, necessary to bias these cathodes positivelyto the same degree as the initial automatic volume control bias.

This may be conveniently done by connecting all the cathode returns ofthe receiver tubes being controlled to a Wire |68 which is connected tothe same positive bias source that was used for the cathode of thevacuum tube 68.

An additional desirable precaution would be to limit the maximumpositive excursion, of the automatic volume control bus, to the cathodepotential of the receiver tubes. This may be done by providing a diodeYrectier within one of the receiver tubes. YThis diode is connectedbetween its cathode and the automatic volume control bus. This idode isnon-conductive when the bus is less positive than the cathode.

Satisfactory loop rotative speeds have been found to be between 30 to 60per second. Trip- Y ping may be readily accomplished at 30 or more` frompoints of maximum response.

The foregoing description has described how automatic direction findingmay be accomplished using only a single antenna.

Due to the well known directional characteristics of a loop antenna twobearings apart are always found. Under the conditions just describedthis 180 ambiguity is likewise experienced in this direction finder.

This uncertainty may be resolved by employing a sense antenna. In thiscase, as in the case of previously used direction finders, the phase ofthe signal received by the Vloop antenna signal is compared to thatreceived by the sense antenna. Now the phase of the sense antenna signalis constant While that of the loop antenna shifts 180 as it rotatesacross the point of no response.

For several reasons it was found undesirable to receive the sense signalcontinuously. The sense signal is only applied momentarily followingeach tripping action.

The non-directional sense antenna |2 is connected to the grid of avacuum tube |10. The grid is biased to ground Athrough a pair ofresistors 200 and |90. The resistor |90.is electively Icy-passed by acondenser |88 so that the effective termination consists of the resistor299. This resistance 209 mustbe large compared to the reactance of theantenna and associated wiring, at the lowest frequency in the operatingrange. This is in order to maintain proper phase relations One-halfmegohm is a suitable value for this resistance.

Suitable supply voltages are established for the plate, screen andcathode by means of resistors |18, |80, |82 and |84. These resistors areof such value that the positive cathode bias will nearly extinguish theplate current. A pair of condensers |12 and |14 effectively by-passthese resistors at radio frequencies.

The signal output from the vacuum tube |10 is transmitted through atransformer |16 to the input of the receiver in parallel with the loopantenna -|0.

This transformer |15 should have a very close coupling between itswindings. The secondary inductance should be several times theinductance of the loop antenna l so as not to unduly affect thetransmission from the latter. The primary, connected to the plate of thevacuum tube |10, will `have a very much lower inductance. Thisinductance .should be just high enough to transmit .sense signals ofdesired strength. Any transmission in excess of this is undesirable. Themagnitude of this signal need not exceed 20% of the maximum signal ofloop antenna I0.

It is evident that normally no sense sig-nal is transmitted as thevacuum tube is normally extinguished. Following each tripping action apulse of short duration is transmitted in the following manner:

vThe cathodes of the vacuum tubes |02 and |04 are connected through acondenser |98 to the, pulse shaping network consisting of the condenser|88, the resistor |90 and a resistor |92 and a pair of reetifiers |94and |96. These rectiers 19,4 and |96 may be .of the germanium crystal'type .and their directions of conductivity are as indicated 'by thearrow. f f

VWhen tripping action occurs-a negative pulse is transmitted from thecathodes of the vacuum tubes |02 and |04 through the condenser |98 andis promptly absorbed by the rectier |96. No significant action takesplace in the circuits of the sense antenna |0. The loop rotating motor|24 slows down and then reverses. Shortly before or after this motorreversal the trigger circuit releases.

his .sends a positive pulse through the condenser |.98, rectifier |94and the resistor v|92 to the condenser |88 which is shunted by the highresistance |90. This charges the condenser |88 `positively and throughthe resistor 200 places a positive voltage on the grid of the vacuumtube '|10 thus rendering it conductive and permitting a sensev signal tobe transmitted to the .receiver .and associated direction findingcircuits.

The sense signal must not be injected into the receiver 22 abruptly. Ifthis is done the limiter .and time delay circuits would prevent itstransmittal to the tripping circuit.

j For this reason, the current charging condenser |88 Tis limited by theresistor |92 in series with the vrectifier -|94. A time constant greaterthan that of the resistor 90 and the condenser 92 is then obtained.

` 'During this charging period the amplication 'of the vacuum tube |10is continuously increased,

The amplitude and l0 rate of this injected sense signal must be such asto be capable of actuating the tripping circuits. After the condenser|88 receives its maximum charge, it begins its discharge through theresistor |90 only the rectifier |94 being non-conduct; ing for currentsin this direction. The discharge resistor .|90 is made muchl larger thanthe chag= ing resistor so that the rate of change .of the falling senseantenna signal will be too small to aotuate the tripping circuits.

Now this momentary .injection of lsense signal may act vin either of.two ways depending upon which of the two .ambiguous bearings the loopantenna is responding.

In the rst way, the injected sense signal will be in phase with the loopantenna signal anda rise in combined :signal will result. This causes nosignificant action. In this case the loop antenna i0 'is oscillatingabout the proper bearing.

In the second way, the injected sense Asignal will be out of phase withthe loop antenna signal. This indicates that the loop antenna :l0 isoscillating about the wrong bearing. In this case a fall in the combinedsignal will result due to their difference in phase. This causes thetripping circuits to be reactuated and the loop rotation, after but a.momentary hesitation, continues towards the point of minimum response.

'I'he first trigger circuit is held tripped by the continuously fallingsignal and usually releases only after the point of minimum response iscrossed. In such a case .no further significant action will take placeuntil the loop antenna I0v rotates beyond the opposite point of maximumresponse and .again enters a region of falling sig.

nal response. Subsequent tripping and sensing actions will then continue.about the proper point of maximum response.

A somewhat different action may result if the automatic volume controlaction is diierent. In this case after the retripping due to the .sensepulse the trigger circuit is initially held in the tripped position asbefore. However, the continuously falling signal must shortly .cause the`automatic volume control to .begin functioning. If the automatic volumecontrol functions rapidly enough, .it may increase the amplincationalmost as rapidly as the signal falls. Insuch a case the trigger circuitwill release which in itself Vwill 4cause no significant action.However, as rotation .continues towards the point of minimum .responseJthe rate of signal change rapidly increases and may again exceed thespeed of .action of the automatic volume control so that anothertripping action will ensue. .In such a case .the .sense cycle isrepeated and, after but a momentary hesitation, loop rotation iscontinued as before. It is thus evident that in either case properlsense is assured.

When severe precipitation static exists, excessive noise is picked up bythe sense antenna .|.2 that will only affect slightly a shielded loopantenna. Under vthis condition the sense -antenna circuit may bedisabled. This will stillggive continuous :automatic direction ii-nding'but rwith ambiguity. This ,may bev accomplished by operation of vaswitch |86. This grounds the sense .antenna l2 and adds -a resistor 202in the cathode bias circuit of the vacuum tube |70. The resistor 202should be large enough to hold the vacuum tube |10 Ycompletelyextinguished dur-ing sense pulses.

All plate voltages may besecured from :a power source 62 which may be abattery, dynamo-tor or .any other convenient source.-

VThe laments of all vacuum tubes may secure power from the source 64which will usually be a storage battery but need not be restricted tosuch. The wiring of the filaments is not illustrated on the drawing assuch methods are well known.

The several positive biases previously mentioned may be secured from thesource 64 but are not restricted to such a source as they could beobtained from the source B2 or some other source without in any wayaiecting theV operation.

' Many obvious variations in the circuit described are possible withoutaffecting its principles of operation. Among these are the following.

Any of. the reotiers may be thermionic diodes or other suitable types.I'he rectier 32 may be separated from the vacuum tube 3U.

Multigrid tubes may replace triodes and vice versa.

Selsyn motors may be used in place of indicator M6 for remote indicationof azimuth. Deviation compensation may be associated with azimuthindication.

The loop rotating motor |24 may be controlled 'by a relay. Saturablecore reactors controlling an alternating current motor could also beused.

Having thus described the invention, what is claimed as new and desiredto be secured by Letters Patent is:

l. In a radio direction finder of the kind described, structure adaptedto receive and transmit radio signals, said structure being rotatable,

whereby the amplitude of the transmitted signals changes as thestructure rotates; automatic means for rotating said structure in eitherof two directions; an electric circuit connected with said structure forreceiving signals therefrom; circuit elements operably connected withsaid electric circuit and responsive to the rate of change in amplitudein said signals for controlling said automatic means; and filteringmeans for rendering said elements less responsive to bursts of staticreceived by said electric circuit.

2. In a radio direction finder of the kind described, structure adaptedto receive and transmit radio signals, said structure being rotatable,whereby the amplitude of the transmitted signals changes as thestructure rotates; automatic means for rotating said structure in eitherof two directions; an electric circuit connected with said vstructurefor receiving signals therefrom; circuit elements operably connectedwith said electric `circuit and responsive to the rate of change inamplitude in said signals for controlling said automatic means; and ltermeans interposed between said electric circuit and said circuit elementsfor delaying the application of changes in -signal amplitude to saidelements, whereby to reduce elects of bursts of static received by saidelectric circuit upon said elements.

3. In a radio direction finder of the kind described, structure adaptedto receive and transmit radio signals, said structure being rotatable,-whereby the amplitude f the transmitted signals -changes as thestructure rotates; automatic .means for rotating said structure ineither of two directions; an electric circuit connected with saidstructure for receiving signals therefrom; 'circuit elements forcontrolling said automatic means; means interposed between said electriclcircuit and said circuit elements for applying voltage to said elementsonly during changes in amplitude of said signals; and lter elements fordelaying the application of said voltage to said 'i elements.

4. In a radio direction finder of the kind described, structure adaptedto receive VandV transmit radio signals, said structure being rotatable,whereby the amplitude of the transmitted signals changes as thestructure rotates; automatic means for rotating said structure in eitherof two directions; an electric circuit connected with said structure forreceiving signals therefrom; circuit elements for controlling saidautomatic means; means interposed between said electric circuit and saidcircuit elements for applying voltage to said elements only duringchanges in amplitude of said signals; lter elements for delaying theapplication of said voltage to said elements; and means for limiting thesaid 'voltage applied to said iilter elements.

5. In a radio direction finder of the kind described, structure adaptedto receive and'transmit radio signals, said structure being rotatable,whereby the amplitude of the transmitted signals changes as thestructure rotates; automatic means for rotating said structure in eitherof two directions; an electric circuit connected with said structure forreceiving signals therefrom; circuit elements operably connected withsaid electric circuit and responsive to the rate of change in amplitudein said signals for controlling said automatic means; and an automaticvolume control interposed in said electric circuit and having partsoperable to control ampliiica-` tion of said signals outside apredetermined limited range.

6*. In a radio direction finder of Ythe kind described, structureadapted to receive and transmit radio signals, said structure beingrotatable,`

cuit elements operably connected with said electric circuit andresponsive to the rate of. change in amplitude in said signals forcontrolling said automatic means; and an automaticvolume controlinterposed in said electric circuit and having parts operable to controlamplification of said signals outside a predetermined limited range,said parts including a pair of rectiers arranged to decrease andincrease the amplification by said electric circuit as the magnitude ofsaid signals increase and decrease respectively. '7. A radio directionnder comprising rotatable structure adapted to receive and transmitradio signals; means connected to said structure for receiving signalstherefrom, said kmeans including an amplifier tube and means to rectifythe carrier frequency voltage of said signals; automatic means forrotating said Vstructure in either of two directions to produce a changein amplitude of the incoming signals; circuit elements responsive to therate of change in ampli- -as the magnitude of the signals increases .and

decreases respectively.

8. In a radio direction iinder of the kind described, rotatablestructure adapted to receive and transmit radio signals; automaticapparatus for rotating said structure in either of two di.- rections toproduce a change in amplitude of the incoming signals; a sense antenna;an electric circuit connected with said structure and said antenna forreceiving signals therefrom; circuit elements operably connected withsaid electric circuit and responsive to the rate of change in amplitudeof said signals for rendering said automatic apparatus operable tonormally oscillate said structure about either of two possible positionswhere the magnitude of the signal received by said structure is atmaximum; and means for applying the signals emanating from said senseantenna to the electric circuit to control said circuit elements onlywhile said structure tend to oscillate about one of said positions.

9. A radio direction iinder of the character described comprisingrotatable structure adapted to receive and transmit radio signals;automatic apparatus having reversible parts for rotating said structurein either of two directions to produce a change in amplitude of theincoming signals; an electric circuit connected with said structure forreceiving signals therefrom; circuit elements operably connected withsaid electric circuit and responsive to the rate of change in amplitudeof said signals for periodically reversing said automatic apparatus; andmeans for rendering said automatic apparatus operable to oscillate thestructure about only one of two possible positions where the magnitudeof the signal received by said structure is at maximum.

10. A radio direction iinder of the kind described comprising rotatablestructure adapted to receive and transmit radio signals; automaticapparatus having reversible parts for rotating said structure in eitherof two directions to produce a change in amplitude of the incomingsignals; a non-directional sense antenna; an electric circuit connectedwith said structure and said antenna for receiving signals therefrom;circuit elements operably connected with said electric circuit andresponsive to the rate of change in amplitude of said signals forperiodically reversing said automatic apparatus; and means includingsaid sense antenna for rendering said automatic apparatus operable tooscillate the structure about only one of two possible positions wherethe magnitude of the signal received by said structure is at maximum,said means having parts for directing signals from said sense antenna tosaid electric circuit only momentarily at each reversal of saidautomatic apparatus.

11. A radio direction finder of the kind described comprising arotatable structure adapted to receive and transmit radio signals;automatic apparatus having reversible parts for rotating said structurein either of two directions to produce a change in amplitude of theincoming signals; a non-directional sense antenna; an electric circuitconnected with said structure and said antenna for receiving signalstherefrom; circuit elements operably connected with said electriccircuit and responsive to the rate of change in amplitude of saidsignals for periodically reversing said automatic apparatus; and meansincluding said sense antenna for rendering said automatic apparatusoperable to oscillate the structure about only one of two possiblepositions Where the magnitude of the signal received' -duce a change inamplitude of the incoming signals; a non-directional sense antenna; anelectric circuit connected with said structure and said antenna forreceiving signals therefrom;

circuit elements operably connected with said electric circuit andresponsive to the rate of change in amplitude of said signals forperiodically reversing said automatic apparatus; and means includingsaid sense antenna for rendering said automatic apparatus operable tooscillate the structure about only one of two possible positions wherethe magnitude of the signal received by said structure is at maximum,said means having parts operable to control said circuit elements onlywhen the signals received by said sense antenna are incorrectly phasedwith the signals received by said structure.

JOSEPH G. SPEER.

REFERENCES CITED The following references are of record in the fue ofthis patent:

UNITED STATES PATENTS Number Name Date Re. 23,066 Moseley Dec. 21, 19482,257,757 Moseley Oct. 7, 1941 2,423,337 Moseley July 1, 1947 2,455,939Meredith Dec. 14, 1948 2,495,591 Meredith Jan. 24, 1950

